This invention relates to disc cutters for rock working machines such as raise borers and boxhole borers.
Conventional disc cutting machines of the type in question have disc cutters which comprise a body journalled for rotation about a shaft secured to the rock working machine. On the body there is a steel cutting ring which is secured with a high interference fit as by heat shrinking. When hard abrasive rock has to be worked, the steel rings on conventional cutting discs blunt very quickly so that rock working machines with cutting discs become uneconomical to use.
It has therefore been proposed to replace the steel cutting edge in such a disc with hardmetal such as cobalt cemented tungsten carbide or titanium carbide. The earliest proposals in this regard dealt with the fixing of segments of hardmetal on the body--see U.S. Pat. Nos. 3,981,370 and 3,982,595. These proposals did not, however, come into practical use, probably because of failures at the abutting faces of the segments.
In U.S. Pat. No. 3,766,998 the use of a continuous ring of hardmetal is proposed. The ring is lozenge shaped in cross-section and is clamped between inclined faces which exert a radially outward hoop stress. The patent cautions against applying too much tensile stress to the ring, but it advocates the use of some amount of tensile stress to start with. This proposal has also not been commercialised and the applicant believes that such a ring would fail in use due to the cumulative effects of the initial hoop stress and hoop stresses caused by the cutting loads. The geometry of the ring support does not have sufficient stiffness to provide adequate support to the ring. The induced tensile hoop stresses in the lower portion of the profile due to cutting loads superimposed on the tensile hoop stresses due to fitting will cause the ring to break.